1. Field of the Invention
The present invention relates to a method for manufacturing an electronic module.
2. Description of Background Art
In particular, the invention relates to an electronic module, which includes one or more components embedded in an installation base. The electronic module can be a module like a circuit board, which includes several components, which are connected to each other electrically, through conducting structures manufactured in the module. The components can be passive components, microcircuits, semiconductor components, or other similar components. Components that are typically connected to a circuit board form one group of components. Another important group of components are components that are typically packaged for connection to a circuit board. The electronic modules to which the invention relates can, of course, also include other types of components.
The installation base can be of a type similar to the bases that are generally used in the electronics industry as installation bases for electrical components. The task of the base is to provide components with a mechanical attachment base and the necessary electrical connections to both components that are on the base and those that are outside the base. The installation base can be a circuit board, in which case the construction and method to which the invention relates are closely related to the manufacturing technology of circuit boards. The installation base may also be some other base, for example, a base used in the packaging of a component or components, or a base for an entire functional module.
The manufacturing techniques used for circuit boards differ from those used for microcircuits in, among other things, the fact that the installation base in microcircuit manufacturing techniques, i.e. the substrate, is of a semiconductor material, whereas the base material of an installation base for circuit boards is some form of insulating material. The manufacturing techniques for microcircuits are also typically considerably more expensive that the manufacturing techniques for circuit boards.
The constructions and manufacturing techniques for the cases and packages of components, and particularly semiconductor components differ from the construction and manufacture of circuit boards, in that component packaging is primarily intended to form a casing around the component, which will protect the component mechanically and facilitate the handling of the component. On the surface of the component, there are connector parts, typically protrusions, which allow the packaged component to be easily set in the correct position on the circuit board and the desired connections to be made to it. In addition, inside the component case, there are conductors, which connect the connector parts outside the case to connection zones on the surface of the actual component, and through which the component can be connected as desired to its surroundings.
However, component cases manufactured using conventional technology demand a considerable amount of space. As electronic devices have grown smaller, there has been a trend to eliminate component cases, which take up space, are not essential, and create unnecessary costs. Various constructions and methods have been developed to solve this problem.
One known solution is flip-chip (FC) technology, in which non-packaged semiconductor components are installed and connected directly to the surface of the circuit board. However, flip-chip technology has many weaknesses and difficulties. For example, the reliability of the connections can be a problem, especially in applications, in which mechanical stresses arise between the circuit board and the semiconductor component. In an attempts to avoid mechanical stresses, a suitable elastic underfill, which equalizes mechanical stresses, is added between the semiconductor component and the circuit board. This procedural stage slows down the manufacturing process and increases costs. Even the thermal expansion caused by the normal operation of a device may cause mechanical stresses large enough to compromise the long-term reliability of an FC structure.
U.S. Pat. No. 4,246,595 discloses one solution, in which recesses are formed in the installation base for the components. The bottoms of the recesses are bordered by an insulation layer, in which holes are made for the connections of the component. After this, the components are embedded in the recesses with their connection zones facing the bottom of the recess, electrical contacts being formed to the components through the holes in the insulation layer. In such a method, problems can arise, for instance, when aligning the feed-throughs with the contact zones of the component This is because the feed-throughs must be aligned relative to components lying under the insulation layer. In other ways too, the method does not correspond to the technology used nowadays (the patent dates from 1981).
JP application publication 2001-53 447 discloses a second solution, in which a recess is made for the component in the installation base. The component is placed in the recess, with the component's contact zones facing towards the surface of the installation base. Next, an insulation layer is made on the surface of the installation base and over the component. Contact openings for the component are made in the insulation layer and electrical contacts are made to the component, through the contact openings. In this method too, the alignment of the feed-throughs with the contact zones of the component can cause problems, as the alignment must be made relative to a component lying under the insulation layer. In the method, considerable accuracy is demanded in manufacturing the recess and setting the component in the recess, so that the component will be correctly positioned, to ensure the success of the feed-throughs, relative to the width and thickness of the installation board.
In general too, the connection of components through feed-throughs made in the insulation layer creates a challenge to techniques, in which an attempt is made to embed components inside a circuit board or other installation base. Problems can arise, for example, due to the alignment precision, the stress created on the surface of the component by the manufacture of the hole, and by the covering of the edge areas of the feed-through by conductive material. Even a partial reduction of the problems relating to feed-throughs would be beneficial to the low-cost manufacture of reliable electronic modules that include unpackaged components embedded in an installation base. On the other hand, embedding a component inside an installation base will allow the construction to better withstand mechanical stress, which has been a problem in flip-chip technology.